The invention relates to a disc brake to be opened by torque, the disc brake being arranged between a drive shaft and a driven shaft and comprising at least one brake wheel arranged on the driven shaft non-rotatably in the peripheral direction; at least one friction surface arranged to cooperate with the brake wheel; at least one spring device arranged to press the brake wheel axially against the friction surface; and means causing, by the action of the torque and torsion of the drive shaft and a possible countertorque of the driven shaft, the brake wheel and the friction surface to drift axially apart and, accordingly, the braking coupling between them to loosen against the spring force of the spring device. Disc brakes of this kind are applied for instance to stop a transfer motion of a crane or a hoisting apparatus or to some other corresponding object not having a separate driving force provided by the force of gravity, for example.
A known disc brake in connection with a self-braking gearing is disclosed in German Patent 197 26 656. It serves as a load brake together with a motor, the speed of rotation of which is not going to an over-synchronized area and is thus not able to brake (to act as a generator). One end of the gearing shaft is then mounted in bearings at the end face of a casing, to which face around said shaft at a distance from and in parallel with it pins are fastened, to which the brake disc is arranged. Friction surfaces on discs supporting them, these discs being arranged on the gearing shaft non-rotatably, but axially movably, are arranged to bear against the surface of each side of the brake disc. The supporting disc situated farthest from the end face of the casing additionally comprises holes having a stepwise diameter, which holes are arranged at the same circumference at a predetermined distance from each other. At a distance from the last-mentioned supporting disc of the second friction surface, there is a pinion of the gearing, positioned axially unmovably, but rotatably on the gearing shaft. This pinion is, in turn, provided with the same number of openings having a conical cross-section as the supporting disc and arranged in the same way. A spring is arranged between the supporting disc provided with holes and the pinion of said gearing, and rocking levers are arranged between the supporting disc provided with holes and the corresponding holes in the pinion of the gearing. A force braking the gearing is provided in such a way that the spring mentioned above presses the friction surfaces of the supporting discs against the brake disc. In turn, the brake is opened by means of said rocking levers. For this purpose, the rocking levers have plate-like head portions dicker than their arm at the supporting disc end of the friction surfaces, the head portions extending through the holes in the supporting disc between the supporting discs and being pressed between them. Now, when the pinion of the gearing is subjected to torque, it begins to turn with respect to the supporting discs of the friction surfaces, whereby the rocking levers are able to move to an oblique position thanks to the design of the holes in the supporting disc and the pinion of the gearing. At the same time, the plate-like head portions of the rocking levers are prizing the supporting discs of the friction surfaces and thus the very friction surfaces apart from each other, so that the coupling which brakes the gearing is opened. Together with still increasing rotational angle between the supporting discs and the pinion of the gearing, the braking force decreases further, and when the drive torque reaches the sum of the residual braking torque and the load torque, the brake will open completely and the gearing shaft may rotate freely in the upwards direction (hoisting direction). The brake will not open in the downwards direction (falling direction), but load torque times rotational angle (rad) turns into heat (E=TLxxcfx86). This restricts the objects of use to smaller devices.
A problem with this solution is exactly this rocking lever arrangement, which opens the brake and is sensitive to wear and damages. The structure causes also noise, which is increased further by the wear. To make the braking and the opening of the brake to operate as desired, it is necessary to check carefully how the friction surfaces wear, because no adjustment can be carried out. Of course, this means as well that the function of the brake changes as per the wearing of the friction surfaces or the brake disc. The solution is also relatively complicated and requires a number of special parts and is not suitable to be used directly in connection with units to be formed of conventional electric motors and gearings.
European Patent Application 0 109 918 discloses a brake actuator, providing a strong pressure force on a pressure plate compressing a disk brake stack for braking of a rotating wheel. The actuator includes a stiff spring working in association with either an electric torque motor or a ball-screw mechanism to achieve an increased torque to a ring gear that drives a ram of the ball-screw mechanism into or out of engagement with the pressure plate, the spring being adapted either to activate or deactivate the braking operation. However, this brake actuator is only intended to intensify the brake influencing the wheels of an aircraft.
In the objects of use described in the beginning, the most conventional manner of switching on and off a brake has been, however, to use electromagnetic actuators, because they have been better than the known brakes to be opened mechanically by torque.
The object of the invention is to start developing, on the basis of the prior art described in the beginning, best represented by said German Patent, such a disc brake to be opened mechanically by torque which could replace separate expensive electromagnetic brakes and which would be better than the prior art brakes in every respect.
This object can be reached by a disc brake in accordance with the invention, which is of the above type to be opened by torque and which is characterized in that the drive shaft and the driven shaft are interconnected in such a way that they may turn a restricted rotational angle with respect to each other and that means for causing said axial displacement and for loosening the braking coupling comprise cam means arranged between the brake wheel and the drive shaft.
The invention is based on the idea to couple the drive shaft and the driven shaft directly to each other, but yet in such a way that they are allowed to turn with respect to each other restrictively, which enables simple and very reliable axial pushing means to be arranged between the drive shaft and the brake wheel.
In comparison with electromagnetic brakes, an advantage of the brake of the invention consists primarily in that losses generally experienced as a drawback, such as for example friction torque of a driven device (at transfer gearing, for instance, resistance to motion of a trolley to be transferred) are utilized for opening the brake. Each time there is enough counter-torque at the driven device, the brake may open. The brake opens always, because, when the driving device (a squirrel-cage motor, for example) is started, it always has to overcome the brake torque, which it experiences as a countertorque. Because no separate magnet is needed for the operation of the brake, it is not necessary either to adapt the brake voltage to the motor voltage in electromotor applications, which is a great advantage. No rectifier is needed either. The opening force of the brake does not depend on the wearing of the friction surface, on account of which the brake does not need to be adjusted. The wear margin of the brake depends on the geometry of the cam means.